1. Field of the Invention
The present invention relates to solid particulate materials that are useful for the reversible absorption of chemical species from a fluid, such as from a liquid or gaseous stream, and particularly the removal of gases such as CO2, H2S or NOx, from a gas stream. Particulate materials useful for the reversible absorption of hydrogen are also provided. The particulate absorbent materials can be produced by spray processing to form a powder batch of absorbent material particulates, or intermediate compounds that can be converted to the absorbent material. The absorbent material particulates can be fabricated into shapes such as extrudates, pellets or monoliths.
2. Description of the Related Art
Solid absorbents are utilized in a wide range of applications to remove a chemical species from a gas or liquid stream. There are several mechanisms by which absorbents can remove the targeted chemical species, including physical sorption, chemical sorption and chemical reaction. The first two categories rely on the surface area and the surface composition of the solid sorbent to react with the targeted species and remove it from the stream. Solid reactive absorbents are chemical compounds that fix the selected chemical species by reacting with the chemical species to form a new compound.
Applications of these absorbent materials include, but are not limited to, the treatment of fuels to purify the fuels and the treatment of combustion gases, such as from a coal-fired power plant or an internal combustion engine, to remove noxious components. The removal of H2S (hydrogen sulfide) is motivated primarily by the desire to reduce environmental pollution. Also, the removal of sulfur-containing species, even to below the trace (ppm) concentration range, is desired prior to the use of gases in other applications, as sulfur can poison various catalysts.
Generally, coal includes sulfur as an impurity, which upon gasification typically enters the coal gas stream as H2S. It is common practice to cool the gas below 77° C. to remove H2S by wet scrubbing. However, this practice reduces the overall efficiency of the power generation plant significantly. Hot gas cleanup methods capable of operating at temperatures close to the gasifier outlet temperature, such as from 725° C. to 1325° C., would increase the overall energy conversion efficiency of the plant. While various absorbents have been proposed for desulfurizing hot coal gas, few are effective at such high temperatures.
It is also known that zinc-based materials such as ZnO (zinc oxide) are highly effective for removing H2S at temperatures below about 650° C. Zinc-based compounds can be combined with other metal oxides (such as Al2O3 or TiO2) to prepare absorbent materials with enhanced performance. U.S. Pat. No. 4,313,820 to Farha et al. describes the combination of zinc oxide with titania and at least one promoter selected from vanadium, chromium, manganese, iron, cobalt, nickel, molybdenum, rhenium and their compounds. U.S. Pat. No. 6,479,429 to Khare discloses a sorbent prepared by mixing a zinc-containing compound with an alumina component, among other components, spray drying the mixture to form particles and then subsequently calcining the mixture to form an enhanced H2S sorbent. Other combinations of metal oxides with zinc oxide are also known in the art, such as combinations with oxides of iron and/or nickel, such as those described in U.S. Pat. No. 5,244,641 to Khare. In certain cases, mixed oxides are prepared with the addition of an activator, such as a noble metal oxide, copper metal, copper carbonate, and others, to enhance the kinetics of the adsorption process. This process is described in U.S. Pat. No. 6,251,348 to Scranton. U.S. Pat. No. 4,729,889 to Flytzani-Stephanopoulos et al. describe regenerable H2S sorbents consisting of mixed oxides of Group VB or VIB metals and Group IB, IIB, VII metals supported on refractory metal oxide supports, such as alumina or zirconia. Similarly, U.S. Pat. No. 4,489,047 to de Jong et al. describes the use of MnOx or iron oxide, supported on a porous carrier consisting of alumina, which may also contain silica. U.S. Pat. No. 4,089,809 to Farrior et al. describes the use of silica supported iron oxide for the removal of H2S from gas streams. The contents of each of the foregoing U.S. patents are incorporated herein by reference in its entirety.
At higher temperatures, calcium-based absorbents are promising because the reaction of CaO with H2S is both thermodynamically and kinetically favorable. These reactions are illustrated by Equations 1 to 4.Absorption: CaO+H2S→CaS+H2O  (1)Oxidation: CaS+2O2→CaSO4  (2)Reduction: CaSO4+CO→CaO+CO2+SO2  (3)CaSO4+H2→CaO+H2O+SO2  (4)
It has been proven by van der Ham et al. (Ind. Eng. Chem. Res., 1996, 35, 1487) that the H2S content of gas produced by an air- and steam-blown coal gasifier operating under typical conditions can be reduced to 20 ppmv by employing CaO in highly reducing conditions at a temperature higher than 800° C. Regeneration can be accomplished at a temperature above the absorption temperature by employing a cyclic oxidation and reduction process as is disclosed in U.S. Pat. No. 6,083,862 to Wheelock.
CO2 (carbon dioxide) is another chemical species that is often removed from certain fluid streams. Several technologies and processes are currently available to separate and capture CO2 from other gases. Separation and capture is often required to: (1) remove CO2 that is either present or produced as a co-product or by-product in industrial processes, such as synthetic ammonia production, H2 production and natural gas processing; and (2) prevent the atmospheric release of CO2 when it is created during the generation of electricity, such as by coal combustion.
With regard to the removal of CO2 in industrial processes, pressure swing adsorption (PSA) is a common commercial process, and utilizes pressure changes to promote the cyclic adsorption and desorption of the gas. Generally, a column packed with a highly porous reversible adsorbent, such as activated carbon or surface modified zeolites is employed.
Other materials and methods used to remove CO2 are described by Wong and Bioletti in “Carbon Dioxide Separation Technologies”, Alberta Research Council, the contents of which are incorporated herein by reference in their entirety. These materials and methods include physical solutions, cryogenic separation, membrane separation and chemical absorption.
Zeolite-based materials have been used for CO2 removal in a variety of applications. For example, NH4+-type or H+-type faujasites, and ion-exchanged-type faujasites, such as with zinc or a rare earth can be used for CO2 removal from a mixed gas of hydrogen, nitrogen and methane in the temperature range of 50 to 100° C., as described in U.S. Pat. No. 4,775,396 to Rastelli et al., the contents of which are incorporated herein by reference in their entirety. X or LSX ion-exchanged zeolites can also be used for cryogenic purification of inert gas from CO2 to less than 10 ppb. These types of materials can also be used for hydrogen purification by removal of CO2 from the reformate through PSA methods. Also, modified zeolites, such as a zeolite impregnated with one or more metal salts can be used for hydrogen production from steam reforming carbon-based fuels, as is described in U.S. Pat. No. 6,565,627 to Golden et al., the contents of which are incorporated herein by reference in their entirety.
Similar to porous zeolites, support materials with high surface area such as alumina or activated alumina, are also known to remove CO2. For example, activated alumina with a high surface area that contains at least 80% alumina oxide, silicon oxide, iron oxides and up to 7.25% of alkali or alkaline-earth metal can be used for removal of CO2 from air prior to cryogenic separation or from a synthesis gas, as disclosed in U.S. Pat. No. 6,379,430 to Monereau, the contents of which are incorporated herein by reference in their entirety. The combination of activated alumina with zeolites can be used for CO2 removal for semiconductor purposes.
Other similar materials include aminated carbon molecular sieves, such as those described in U.S. Pat. No. 4,810,266 to Zinnen et al., the contents of which are incorporated herein by reference in their entirety. Another class of materials are the surface functionalized carbonaceous absorbents described in U.S. Patent Application Publication No. 2002/0056686 to Kyrlidis et al., the contents of which are incorporated herein by reference in their entirety, wherein the organic group attached to the surface of the particle comprises one or more amines.
Reactive absorbent compounds are chemical compounds that fix a selected chemical species by reacting with the chemical species to form a new compound. An example is the reaction of a metal oxide (MyOx) with CO2 to form a carbonated compound (MyCO3), as is illustrated by Equation 5 for calcium oxide and Equation 6 for lithium oxide:CaO+CO2→CaCO3  (5)Li2O+CO2→Li2CO3  (6)
In many applications, it is desirable after absorption to regenerate the absorbent by desorbing the CO2 from the carbonate compound. This can be accomplished, for example, by heating the carbonate compound.
A variety of reactive absorbent compounds are available for the absorption of CO2, and each has a different absorption capacity. Chemically reactive CO2 absorbent compounds are preferred, because these compounds typically have much higher equilibrium absorption capacity than other absorbents, as is illustrated in Table 1.
TABLE 1Absorption Capacity(grams CO2 per 100Materialgrams material)methylethanolamine6(MEA)silica gel1.32activated carbon8.8K2CO3/Hydrotalcite1.98(HTC)CaO78.57
In general, alkali metal and alkaline earth metal oxides and/or hydroxides are good materials for CO2 sorption. Reaction of CO2 with various metal oxides leads to the formation of the corresponding carbonates, as described by the following equations:M(OH)2+CO2→MCO3+H2O  (7)MO+CO2→MCO3  (8)2MOH+CO2→M2CO3+H2O  (9)M2O+CO2→M2CO3  (10)
Such reactive absorbent compounds can be freestanding, mixed with or supported on inert porous substrates, such as silica (SiO2), alumina (Al2O3), carbon, and the like. Known absorbent materials include calcium-based compounds, such as calcium hydroxide (Ca(OH)2) and calcium oxide (CaO), magnesium-based compounds, such as magnesium hydroxide (Mg(OH)2) and magnesium oxide MgO, and lithium-based compounds such as lithium hydroxide Li(OH) and lithium oxide Li2O. These absorbents can be enhanced for certain applications with the addition of alkali metal carbonates (M2CO3, where M is an alkali metal) or bicarbonates (MHCO3, wherein M is an alkali metal), at various stoichiometries, as described in U.S. Pat. No. 6,280,503 to Mayorga et al., the contents of which are incorporated herein by reference in their entirety. In other applications, such as the removal of CO2 from air for anesthesiology applications, the presence of alkali metal hydroxides can be detrimental. For these applications, the absorbents should be alkali free, as described in U.S. Pat. No. 6,228,150 to Armstrong et al., the contents of which are incorporated herein by reference in their entirety.
Other materials that have been used for the sorption of CO2 include mixtures of metal oxides, alkali carbonates and alkali fluorides, such as those described in U.S. Pat. No. 5,214,019 to Nalette et al., the contents of which are incorporated herein by reference in their entirety. Nalette et al. disclose that the metal oxide may be selected from the group consisting of MgO, AgO, ZnO and mixtures thereof and that these absorbents can be freestanding, mixed with or supported on an inert porous support. Similar mixed metal oxide absorbents are described in U.S. Pat. No. 5,186,727 to Chang the contents of which are incorporated herein by reference in their entirety. These mixed metal oxide absorbents consist of mixtures of a salt of silver metal with a salt of a second metal selected from magnesium, iron, cobalt, nickel, zinc and other metals for which the carbonate to oxide reaction is reversible. Chang discloses that the preferred metal salt is the carbonate or bicarbonate salt.
Iron oxide based absorbents for the absorption of CO2 are also known in the art. Such materials have applications in the food industry, such as in the packaging of coffee and/or the removal of CO2 from containers that hold respiring fruits and vegetables, as described by Brody et al. in “Active Packaging for Food Applications”, CRC Press, the contents of which. are incorporated herein by reference in their entirety. Solid CO2 absorbents can also be combined with noble metal oxidation catalysts for the removal of trace amounts of CO and H2 from gas streams by concurrent oxidation and sorption, as described in U.S. Pat. No. 6,589,493 to Hosaka et al. and U.S. Pat. No. 6,113,869 to Jain et al, the contents of each being incorporated herein by reference in their entirety.
Another class of sorption materials effective for CO2 removal for both synthesis gas and effluents are porous, solid materials such as mixed oxides of lithium and silicon and/or zirconium. For example lithium zirconate (e.g., LiZrO2) and lithium silicates (having the general formula LixSiyOz) as is described in U.S. Pat. No. 6,387,845 by Masahiro et al., the contents of which are incorporated herein by reference in its entirety, are examples of such materials. It is disclosed that these materials can also incorporate other dopants to enhance their performance, such as Al, K, Fe, Mg and the like, and that the lithium-based materials are reversible upon the application of heat. While the use of lithium zirconate is more widespread at present, the adoption of lithium silicate is increasing due to its lower production costs, lighter weight and rapid CO2 absorption capabilities. For example, one gram of lithium silicate is capable of absorbing 62 milligrams of CO2, making the material 30 times more efficient than lithium zirconate. Lithium silicate is also 70 percent lighter and about 85 percent less expensive than lithium zirconate, since it uses silicon instead of the more expensive zirconium as a starting material.
Despite the breadth of existing and potential solutions for the separation and capture of CO2, there is still the need for improvements in the areas of capacity, high-temperature operation and long-term stability. Indeed, the need for such absorbents is even more pronounced with regard to the removal or collection of CO2 present in the flue gases of fossil-fueled power plants, chemical plants and engines, the presence of which is a contributing factor in global warming and ocean acidification. The increasing use of fossil fuels to meet energy needs has led to higher CO2 emissions into the atmosphere and CO2 emissions from direct combustion of fossil fuels account for one-half of the greenhouse effect that causes global warming. It is therefore desirable to develop cost-effective CO2 management schemes to curb CO2 emissions. Many CO2 management schemes consist of three parts: separation, transportation, and sequestration. The capture of CO2 accounts for about 75% of the total cost of CO2 management, and imposes a severe energy requirement on fossil fuel-based power plants, reducing their net electricity output by as much as 37%. The costs associated with current CO2 separation technologies necessitate the development of economical alternatives. It is believed that, none of the preceding materials and methods have been applied at the scale required for a commercial CO2 emissions mitigation strategy that also meets the associated cost and safety requirements. As such, techniques are needed to transform absorbed CO2 materials into materials that can be economically and safely disposed, can be transported and sequestered for a long time, or can be used to make commercial products that can offset the associated costs of capture and transport. One potential solution is the carbonation of silicate rock, where CO2 is captured in a stable and solid form for disposal. Preliminary estimates for silicate carbonation are in development, but show the potential for a significantly lower cost than solvent extraction.
In addition to the above, several smaller-scale applications exist for the removal of CO2. Specifically, small scale applications exist for enclosed environments, whose elevated concentrations of CO2 are potentially dangerous, such as submarines, space systems, anesthesia machines and diving rebreather equipment. CO2 absorbents can also be used in the removal of trace concentrations of CO2 from air prior to cryogenic separation applications, or removal of acid gases for the preparation of ultra-pure inert gases for semiconductor and other high purity applications. CO2 absorbents are also used as components of CO2 sensors due to the change in their physical properties as they absorb CO2. In addition, CO2 absorbents can help remove CO2 from sensors of other gases, when the adsorption of CO2 interferes with the sensing of the other target gas. In general, CO2 absorbents can be utilized in any application where it is important to control the presence, concentration or release of CO2, and includes chemical applications and applications related to cell culture and/or fermentation processes.
Natural CaO-based absorbents for the absorption of H2S and CO2, such as limestone and dolomite, are plentiful and inexpensive, but they are soft, friable and do not stand up well to handling and use for multiple cycling. To improve- the recyclability, some work has focused on the pelletizing of limestone by using different binders. Work also focused on the modification of natural materials, such as dolomite, to tailor the physicochemical properties of the material. The synthesis of a CaO-based absorbent by boiling of CaO into Ca(OH)2 or the carbonation of calcium salt solution such as calcium nitrate or Ca(OH)2 into calcium carbonate, and subsequent decomposition of the carbonate into CaO is disclosed by L. S. Fan et al., Ind. Eng. Chem. Res., 1999, 38, 2283. Others have disclosed the preparation of CaO-based materials by aerogel methods. Other various materials useful for the absorption of H2S are disclosed in U.S. Pat. No. 4,729,889 to Flytani-Stephanopoulos et al., the contents of which are incorporated herein by reference in their entirety.
Generally, the above described methods result in poor control over the composition and microstructure of the powders. The morphology and surface properties such as surface area, pore volume and pore size, are characteristics that impact the performance of the absorbent. This is primarily due to the nature of the reactions that occur. With respect to CO2, carbonation takes place on the external and internal surfaces of the CaO-based absorbent, thereby forming a carbonate layer. As the chemical reaction advances, CO2 diffuses through the carbonate layer into the unreacted core CaO active sites. Therefore, higher reactivity and faster kinetics can be expected for small particle size CaO due to the higher ratio of atoms on the surface. A more porous structure will also lead to higher reactivity and recyclability, and a lower decarbonation temperature due to the easier CO2 diffusion into and out of the outer carbonate layer.
Absorbent materials that can reversibly absorb and/or store NOx (nitrogen oxides) are also known. The emission of NOx (e.g., NO, NO2, etc.) is detrimental to the environment. It has been shown that NO, can lead to the production of pollutants, such as ozone and can also lead to the formation of acid rain. One primary application for NOx sorbent materials is the removal of nitrogen oxides from exhaust gases. Exhaust gases generally are the products of the combustion of automotive fuels, e.g., long-chained hydrocarbons. The EPA estimates that 50% of NOx emissions are from mobile sources (e.g. automobiles) and about 30% are from stationary sources (e.g. electric power plants). Preferably, NOx is removed at the place of generation (in the vicinity of the engine) or from enclosed spaces, such as tunnels, garages, or other enclosed structures, where the gases may accumulate over time.
There are several other applications that would benefit from NOx absorption, including pickling operations in steel mills and silicon processing. In addition, chemical plants that produce nitric acid or that utilize nitric, or nitrous acids as reagents are additional sources of NOx emissions that could benefit from NOx absorbents. NOx present in air can also interfere with the cryogenic separation of oxygen and nitrogen and the manufacture of ultrapure gases for specialty applications (i.e. semiconductor processing) as described in U.S. Pat. No. 6,358,302 to Deng et al., the contents of which are incorporated herein by reference in their entirety.
Nitrogen oxides are also used in combination with oxygen and anesthetics in various medical applications, such as laughing gas. It is important in these applications to controllably release such gases during the anesthesia and to capture any fugitive gases.
Nitrogen oxides can also interfere with the operation of a variety of devices. For example, copying machines, such as those described in U.S. Pat. No. 5,539,205 to Reale, can generate nitrogen oxides that need to be abated in order to maintain reliable performance during operation of the device. Printed circuit boards, hard drives, or other computer hardware components that are operated in harsh environments containing acid gases, such as nitrogen oxides, can also suffer significant deterioration in performance in the presence of NOx.
An additional application for NOx absorbent materials is in the area of sensors. The performance of sensors for other gases can be enhanced by removing nitrogen oxides from gas mixtures, if they interfere with the performance of the sensing material for the other components of the gas mixture.
The abatement of NOx can be achieved using various methods, (see for example EPA Technical Bulletin 456/F-99-006R), some of which interfere with known mechanism of NOx production, and others which help with the abatement of the NOx species after they are formed.
A variety of materials have been developed over the years for the absorption of nitrogen oxides. The composition and performance of these materials is strongly a function of the conditions under which they are expected to act as absorbents. Each material performs best in a specific environment. In some applications selection of a NOx absorbents is a function of its regenerability. An additional factor is the, potential for combining these materials with a catalyst for reducing the nitrogen oxides to inert nitrogen. Huang and Yang (Langmuir, Vol. 17, (2001), pp. 4997-5003) and its references list a variety of these materials, the contents of each being incorporated herein by reference in their entirety.
Alkali metal, alkaline earth, or lanthanide oxides, hydroxides, and/or carbonates can trap nitrogen oxides through the mechanism:NO+0.5 O2→NO2  (11)MO+2NO2+0.5 O2→M(NO3)2  (12)MCO3+2NO2+0.5 O2→M(NO3)2+CO2  (13)The metal (M) can be selected depending on factors such as cost, desired capacity, and operating conditions. Typical metals include sodium, potassium, calcium and barium. Such materials may be used alone or in combination with an inert support, such as alumina, silica, as described for example in U.S. Pat. No. 4,755,499 to Neal et al., the contents of which are incorporated herein by reference in their entirety. Such inert materials may provide a substrate with the desired porosity, pore size distribution, and surface area to support the active sorbent.
Other materials that have been found to have enhanced sorption capacity for NOx are mixed oxides of manganese (MnOx) and zirconium (ZrO2) (as described by Eguchi et al., J. Catal. Vol. 158, (1996), p. 420, the contents of which are incorporated herein by reference in their entirety). The performance of these mixed oxides can be enhanced by the incorporation of additional metal oxides, such as oxides of titanium and/or iron.
Compositions including Y—Ba—Cu—O mixed oxides have also been shown to have is enhanced storage capacity for nitrogen oxides. For example, U.S. Pat. No. 6,379,432 to Matacotta et al., the contents of which are incorporated herein by reference in their entirety, describes a Ba2Cu3O6 sorbent whose activity can be enhanced with the incorporation of oxides of La and/or Ce. Arai et al., Catalysis Today, Vol. 22 (1994), pp. 97-109, disclose the sorption of nitrogen oxides on YBa2Cu3Oy, which has been shown to have good sorption capacity for NO. These materials can also be supported on inert porous supports, such as those described above. Copper oxides supported on titanium dioxide and copper oxides supported on titanium dioxide, which incorporates cerium oxides, have also been shown to have enhanced performance by Li et al. (Energy & Fuels, Vol. 11, (1997) pp. 428-432), the contents of which are incorporated herein by reference.
The beneficial effect of cerium dioxide for the sorption of nitrogen oxides is also described by Haneda et al. (Phys. Chem. Chem. Phys., vol. 3, (2001), pp. 4696-4700), the contents of which are incorporated herein by reference in their entirety. It is disclosed that the performance of a CeO2:ZrO2 mixed oxide prepared by sol-gel processing was found to be significantly better than that of a similar material prepared by a co-precipitation method.
Other materials that have been shown to sorb nitrogen oxides include amino acid and amine impregnated porous carriers, such as those described in U.S. Pat. No. 6,171,372 to Ichiki et al., the contents of which are incorporated herein by reference in their entirety. Microporous carbons modified by Fe2O3 have also been shown by Inai et al., (Catal. Lett., Vol. 20 (1993), pp. 133-139) to absorb NO from mixtures. Microporous carriers with dispersed nanoparticles that have affinity towards nitrogen oxides can enhance the capacity for adsorption of such materials. Alkali, alkaline earth, rare earth and transition metal cation exchanged zeolites have also been explored as nitrogen oxide sorbent materials. Other materials that have been evaluated include perovskites (ABO3, where A=Ca, Sr, Ba and B=Sn, Zr, Ti), including BaSnO3.
Absorbent materials can have multi-functional components. For example, several of the mixed metal oxides listed above have components with very specific functions: a component that helps oxidize NO to NO2 and a component that stores the nitrogen oxides either on its surface or in its bulk by reaction to the corresponding metal nitrate. Such absorbent materials can also be used as supports for noble metal catalysts that catalyze the oxidation of NO to NO2 and reduce of absorbed nitrogen oxides to nitrogen. Supporting a noble metal catalyst on the sorbent material can significantly enhance the kinetics and overall conversion of the NOx reduction reaction.
All of the materials described above expand their volume upon absorption of NOx during the sorption process. The molar volumes of the nitrates are higher than those of the starting oxides, carbonates, or hydroxides, leading to a weakening of the materials during the sorption process, as they expand. After a few regeneration cycles such materials will become brittle, and, possibly, crumble into finer particles or fall off the support structures. Smaller particles can clog pores, sinter, and/or have significant negative effects on the sorption capacity and kinetics after several cycles. For materials that absorb nitrogen oxides into their molecular structure, as well as for materials that adsorb nitrogen oxide molecules on their surface, it is important to maintain absorbent capacity throughout the use of the sorbent material. Thus, absorbent powders need to be able to maintain their pore structure and surface area even after repeated absorption and desorption cycles.
Certain absorbent materials can also reversibly store H2 (hydrogen). H2 storage materials are normally categorized into three different classes according to their composition and the mechanism for hydrogen storage (e.g., chemisorption or physisorption). Each class of materials has unique properties in terms of the environment in which hydrogen adsorption and desorption occurs, which in turn determines the operating range and possible applications for the hydrogen absorbent.
One type of hydrogen storage material is a metal alloy or intermetallic compound that includes a misch metal, often referred to as a misch metal hydride. Typical misch metal compositions include AB, AB2, AB3, AB5 and A2B, where A can be selected from lanthanide elements (e.g., La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Th, Yb and Lu) as well as Mg, Ti and Zr, and B can be selected from the transition elements (e.g., Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb, Mo, Rh, Ru, Pd, Ag, Cd, La, Ce and the like). See Zuttel, Materials Today, September 2003, pp. 24-33, which is incorporated herein by reference in its entirety. Preferred examples of such materials include LaNi5, Mg2Ni, Mg2Fe, TiFe, and ZrMn2. For example, LaNi5 forms a species with the empirical formula LaNi5H6.5 and is the material of choice for nickel/metal-hydride batteries. Other examples are given in G. Sandrock, Journal of Alloys and Compounds 293-295 (1999) 877-888, which is incorporated herein by reference in its entirety. These materials have very reliable reversible hydrogen uptake and are currently in use in nickel-metal hydride batteries. These materials are also commonly used for hydrogen storage, but have a low gravimetric hydrogen capacity, between 1 and 3 wt. %. In addition, these materials typically decrepitate after the first adsorption/desorption cycle to form a powder that is pyrophoric when exposed to air.
Another class of hydrogen storage material is referred to as chemical hydrides. Chemical hydrides are stoichiometric chemical compounds, typically molecular or oligomeric, but which can stoichiometrically, reversibly react with, or release, H2. In order to achieve a suitable gravimetric H2 storage capacity, these materials usually include hydrides of lighter elements, such as Mg, B, Al, Li, Na, or complexes thereof, including NaBH4, AlH3, LiAlH4, Mg(AlH4)2. These materials generally have H2 storage capacities of up to 9 wt. %. For example, NaAlH4 reversibly reacts to form 1/3 NaAlH6+2/3 Al+H2, which can further reversibly react to form NaH+Al+3/2H2. The theoretical hydrogen storage capacity for this reaction is about 5.6 wt. %. Another promising material is LiBH4, which has a relatively high gravimetric hydrogen density (≈18 wt. %), as described by Zuttel, supra.
In the same class as chemical hydrides are some specific metal hydride compositions which form a known alloy phase on loss of H2. An example of such as compound is Mg2NiH4, which forms 2H2 and Mg2Ni alloy. Another example is Mg2FeH6, which has the potential for 5.5 wt. % H2 storage.
In general, it is believed that the addition of dopants or catalysts can enhance the storage capacity, kinetics, and regenerability of most chemical hydrides. For example, in the case of complex metal hydrides, the addition of a Ti catalyst to sodium alanate (NaAlH4) as described by Bogdanovic and Sandrock (MRS Bulletin, September 2002, pp. 712-716), leads to an increased reversible capacity at 150° C. In the case of simple metal hydrides, the incorporation of Nb2O5 and other metal oxides into Mg can have a significant effect on the adsorption and desorption kinetics of MgH2. See Barkhordian et al., (Scripta Materialia, 49, (2003), pp. 213-217).
Other materials that can- store hydrogen are alkali metal nitrides and imides, especially lithium-based compounds such as Li3N. See Chen et al. (Nature, 420, (2002), pp. 302-303).
A third class of hydrogen storage materials are those that physisorb hydrogen, which are typically highly microporous nanostructured materials. A range of such materials is described by Nijkamp et al. (Applied Physics A, 72, (2001), pp. 619-623), the contents of which are incorporated herein by reference in their entirety. These materials are usually inorganic carbon, silica or alumina based materials with high pore volumes, such as activated carbons, zeolites and others. Organic/metalorganic materials tailored nanostructures, are also known.
The most promising hydrogen absorbing materials include carbon particles and carbon nanotubes/fullerenes. These materials may also include hetero atoms which enhance the hydrogen uptake. Such carbon-based materials can also have surface functionalization groups that enhance the capacities and kinetics of hydrogen storage. High surface area active carbons are known to physisorb molecular hydrogen, but only at low temperatures due to the weak nature of the physisorption interaction. Conversely, chemical reaction of hydrogen with carbon (chemisorption) in the form of fullerenes to form hydrocarbons, e.g., C60H48, results in the formation of covalently bonded hydrogen that requires high temperatures to desorb the hydrogen. To resolve this dichotomy, a number of solutions have been explored. A reduction in the chemical stability of the carbon materials may bring the adsorption/desorption kinetics closer to room temperature. Also, single wall carbon nanotubes have dimensions that are close to that required for capillary condensation of hydrogen molecules and may offer an alternative strategy. Finally, the incorporation of metal particles into the structure of the carbon particles may provide another mechanism to bring the reaction conditions closer to more commercially viable conditions. A recent example of such materials is described in U.S. Patent Application Publication No. 2002/0096048 by Cooper et al., which is incorporated herein by reference in its entirety.
Reversible hydrogen storage materials also include metals, such as Pt, or metal alloys, mixed with or dispersed on the surface of carbon particulates. The carbon particles may also be modified on their surface with organic functional groups to enhance their absorption capability in a number of different ways. Surface. modification helps to selectively bind a catalytically active material, such as a molecular metal-containing complex or a nanometer-sized catalytically active particle, to the surface of the carbon particle. Modified carbon blacks comprising metal particles are described in U.S. Pat. Nos. 6,399,202, 6,280,871, 6,630,268, 6,522,522, U.S. Patent Application Publication No. 2003/0017379 and U.S. Patent Application Publication No. 2003/0022055, each of which is incorporated herein by reference in its entirety. Surface functional groups also affect the uptake of gaseous species, such as hydrogen, by changing the packing characteristics of the carbon particles, as well as the carbon surface characteristics to be, for example, hydrophobic or hydrophilic. Typical surface function groups include carboxylic acids, sulfonyllic acids, amines and the like. A method by which the surface of the carbon particles can be modified is through reactions with diazonium salts of the desired organic functional groups, as described by Belmont et al. in U.S. Pat. Nos. 5,851,280, 6,494,946, 6,042,643, 5,900,029, 5,554,739 and 5,672,198, each of which is incorporated herein by reference in its entirety.
There are also other types of materials, which have been demonstrated to exhibit hydrogen storage properties. These include a number of carboxylate compounds such as zinc acetate.
There are several aspects to the design of hydrogen storage materials which must be successfully addressed in order for such materials to be commercially viable. First, such materials must have a reasonable reversible hydrogen storage capacity. The US DOE target for automotive applications is currently 6.5 wt. % reversible hydrogen storage. Second, the hydrogen storage materials must also exhibit a reproducible capacity over many cycles of hydrogen uptake under conditions of temperature and pressure, which are preferably close to room temperature and pressure. Further, the materials must also have good low-temperature kinetics and equilibrium plateau pressures. They must perform well and be reversible under the highly exothermic charging steps and must be incorporated in systems with good thermal management. In addition, like most sorbents, materials that chemically store hydrogen expand during the adsorption step, and become embrittled. Ideally, the materials should remain intact during the course of the reversible hydrogen uptake and avoid decrepitation, which can have serious operational and safety issues.
It would be advantageous to provide a method for producing particulate absorbent materials or intermediate compounds capable of being converted to absorbent materials that would enable control over the powder characteristics such as particle size, surface area and pore structure, as well as the versatility to accommodate compositions which are either difficult or impossible to produce using existing production methods. It would be particularly advantageous if such powders could be produced in large quantities on a substantially continuous basis. Further value can be derived from these powders if they can be incorporated into structures that can be integrated into reactor beds that enable a suitable combination of high space velocity and high absorption capacity while retaining their performance characteristics. Such structures include coatings such as wash coatings on highly porous monoliths, pellets that have pore structures that retain the performance of the powders, and also coatings or impregnation of the powder particles into other structures, such as metal cloths which provide beneficial heat transfer characteristics.